For the purposes of this specification, an electrochromic display device is a device wherein the display effect is achieved as a consequence of a redox reaction caused by the passage of a charge between a display electrode and a counter electrode, both in contact with a suitable electrolyte. In examples of a conventional electrochromic display is given in British Pat. No. 1,376,799 wherein the electrolyte includes heptyl viologen dications as the color forming material. Upon the reduction at the display electrode, a purple film which is insoluble in the electrolyte is deposited on the display electrode, that is, the writing step. Oxidation at the display electrode returns the dications to the electrolyte, that is, the erasing step. Thus, by providing a plurality of display electrodes and selectively energizing them, a required display can be generated. Other materials can be used. Development has taken the place of displays which employ redox reactions in organic materials such as tungsten oxide, to achieve the display effect.
The successful operation of electrochromic display devices depends upon the redox reaction of the electrolyte which deposits an insoluble nontransparent material on the display electrode as well as removing that deposited material by a reverse reaction. A very common problem with electrochromic displays involves the migration of the electroactive material, that is, deposited material, from the display electrode to the counter electrode.